U.S. President Barack Obama has urged Russia to stop bombing “moderate” rebels in Syria during a conversation with Russian President Vladimir Putin.

The two leaders spoke by phone on February 14, two days after major powers agreed a limited cessation of hostilities in Syria, although none of the warring parties signed the deal that does not take effect until the end of this week.

The White House said Obama's discussion with Putin stressed the need to rush humanitarian aid to Syria and contain air strikes.

"In particular, President Obama emphasized the importance now of Russia playing a constructive role by ceasing its air campaign against moderate opposition forces in Syria," the White House said in a statement.

Russian bombing raids directed at rebel groups are helping the Syrian army to achieve what could be its biggest victory of the war in the battle for Aleppo, the country's largest city and commercial center before the conflict.

According to the Kremlin, Putin and Obama agreed to intensify cooperation to implement the Munich agreement.

But a Kremlin statement made clear Russia was committed to its campaign against Islamic State and "other terrorist organizations", an indication that it would also target groups in western Syria deemed moderate by the West.

Russia says the "cessation" does not apply to its air strikes, which have shifted the balance of power towards Assad.

It says Islamic State and the Al-Qaeda-affiliated Nusra Front are the main targets of its air campaign. But Western countries say Russia has in fact been mostly targeting other insurgent groups, including some they support.

Reaction from politicians in the West to the deal reached in Munich on February 12 was skeptical.

U.S. Senator John McCain said he did not view the deal as a breakthrough. "Let's be clear about what this agreement does. It allows Russia's assault on Aleppo to continue for another week," he said at security conference in Munich.

"Mr Putin is not interested in being our partner. He wants to shore up the Assad regime," McCain said.

A senior ally of German Chancellor Angela Merkel said Russia had gained the upper hand in Syria through armed force.

Norbert Roettgen, head of the foreign affairs committee in the German parliament, said Russia was determined to create "facts on the ground", to bolster its negotiating position.

With reporting by RFE/RL's Steve Gutterman in Munich, AFP, and Reuters